An imaging apparatus can either form the image or read the image or a combination thereof. An image reading apparatus can include a scanner, a spectrophotometer and the like wherein an already formed image is read by the apparatus. An image forming apparatus can include a printer or other graphic arts apparatus. In an imaging apparatus, the component which forms the image or reads the image can be affected by the distance to the image and the way such image is read or formed.
For example, typical printers include inkjet printing systems having a printhead with a plurality of orifices or nozzles, an ink supply which feeds ink to at least one inkjet printhead located on a printhead carriage, and an electronic controller. The printhead carriage passes back and forth over the media supported by a platen selectively depositing ink through its nozzles on the media.
In an imaging apparatus such as an inkjet printer, print quality is highly dependent on accurate ink droplet placement on the print media. Droplet placement accuracy is required to provide minimal graininess and banding, maximum sharpness, line acuity and smoothness. The accuracy of the droplet placement depends on several factors such as consistent spacing between the printhead nozzle and the media over the full length and breadth of the printer platen, especially in bi-directional printing. Deviations in this spacing affect the ink droplet flight time resulting in inaccurate drop placement, and thus reduce image quality.
One source of printhead nozzle and media spacing variation can arise from printhead carriage guide rails with an unacceptable straightness or defect. The primary guide rail for a printhead carriage is typically made with tight tolerances to attempt to maintain a precise and consistent spacing between the printhead and the print media. The secondary guide rail, or anti-rotation rail, can consist of bent sheet metal, in order to decrease the expense of providing expensive pre-straightened rails. In some applications the secondary guide rail can be a discrete elongated member that is mechanically attached to a support structure of the printer. In other applications the secondary guide “rail” may not be a separate part, but can in fact consist of a guide surface on an elongated part of the printer which is spaced apart from the primary guide rail. The terms “secondary guide rail” and “secondary guide surface” will be used interchangeably herein to refer to either type of structure.
Guide rails are more difficult and expensive to produce with adequate straightness in larger printers, such as a wide format inkjet printer, due to their larger size, such as 24 inches or 44 inches wide or wider. In a wide format inkjet printer, waviness in the secondary guide rail can be due to manufacturing processes or can be due to stresses arising from the mechanical attachment of the secondary guide rail to the printer. Such waviness can be cyclic—i.e. there can be recurring peaks and valleys where the peak to peak distance can be described by a wavelength. Dents or defects due to manufacturing and mishandling of the rail produce non-cyclic or single defects in the secondary guide rail.
As the carriage and printhead pass over the various defects, the spacing between the printhead nozzle and the media is affected resulting in inaccurate drop placement, and thus reduce image quality.
In the prior art, a single point of contact is provided between the carriage and the secondary or anti-rotation rail to minimize print defects. For example, U.S. Pat. No. 6,082,854, issued to Axtell et al. discloses a carriage adapted for riding on an anti-rotation rod which acts like an anti-rotation rail and a slider bar which acts like a primary guide rail. A single idler wheel rides atop the anti-rotation rod.
Additionally, U.S. Pat. No. 6,231,160, issued to Glass discloses a pair of bearings that slidably support the carriage on a slider rod, which acts as a primary guide rail. A slide bushing is attached to the rear wall of the carriage. The slide bushing engages an anti-rotation guide bar, which acts like an anti-rotation rail.
U.S. Pat. No. 6,520,622, issued to Yusef et al. discloses a wear device that provides a contact or anti-rotation surface that slidably interacts with the support surface of the anti-rotation rail.